There have been known optical pickup apparatuses having compatibility for a high density optical disc wherein recording density has been enhanced by using a violet laser light source, DVD (employing a red laser light source) and CD (employing an infrared laser light source), and an objective optical element that is used for the optical pickup apparatus above (for example, see non-Patent Document 1).
In the non-Patent Document 1, there is disclosed a technology wherein a phase structure is provided on one optical surface of an objective optical element of a two-group two-element type composed of a plastic correcting element and a glass light-converging optical element. Thereby, a function of the phase structure corrects spherical aberration caused by a protective layer thickness difference between a high density optical disc and DVD.
(Non-Patent Document 1) “TECHNICAL DIGEST”, INTERNATIONAL SYMPOSIUM ON OPTICAL MEMORY 2003 (ISOM 2003), Nov. 3-7, 2003 P230, 231.